The present invention relates to a roll-fusing apparatus comprising a pressure roller and an internally heated fusing roller for fusing a toner image on a copy support, wherein the pressure roller comprises a coated hollow roller core supported on a shaft.
European Pat. No. 17,092 discloses a roll fuser of this type, which is equipped with a pair of rotatable rollers, one of which has end portions which are tapered in the direction of the outer ends. The roller comprises an outer shell which is centrally supported on a shaft. The main parts of the inner surface of the shell are provided with stepped portions of increasing diameters at each tapered end of the shell. In the tapered end portions of the shell, plugs are mounted on parts of the shaft, which are axially displaceable by means of screw threads. The main body of each plug has a diameter which is smaller than the portion of the internal surface of the shell. The plugs are rotatable on the shaft and can be moved from a retracted position, in which a clearance exists between the plugs and the shell of the roller, into an engaged position, in which end flanges of the plugs engage with the stepped portions of the shell. In the retracted position of the plugs, the end portions of the shell are not mechanically supported by the flanges of the plugs and, as a consequence, the pressure exerted by the back-up roller forces the tapered end portions of the shell against the plugs. The amount of taper and the size of the gap between the plugs and the shell are such that the behavior of the roller under the action of contact pressure corresponds to that of a substantially cylindrical roller, i.e., a roller which is not tapered.
The tapered configuration of the roller is maintained, on the other hand, when the end plugs are screwed inwardly in the axial direction. Under the action of pressure contact, a resulting concave contact area leads to a condition in which wrinkling of the copy paper in a high-humidity state is precluded.
When the end plugs are screwed outwardly in the axial direction, the roller operates in the manner of a substantially cylindrical roller which does not have a taper. As a result of the contact pressure which is exerted by the back-up roller, the tapered configuration of the roller is flattened. The application of the roller under this condition prevents the so-called "smearing effect" on the copy, which occurs in a dry state, i.e., at very low humidity. The position of the plugs can be manually adjusted when the two rollers are out of contact or it can be adjusted by means of a motor which is under control of a humidistat.
By supporting the tapered external zones, a higher circumferential speed is produced close to the edges of the roller, so that a copy paper which passes through the gap between the two rollers is conveyed at a higher peripheral speed along its edges, compared to the speed in the middle. As a result, the copy paper is stretched and does not form wrinkles, even at higher humidity. Under very dry conditions, the higher speed in the external zones causes the middle of the copy paper to form a buckle, so that the toner image, which is to be fused, prematurely contacts the fusing roller, shortly before it enters into the fusing apparatus, and thus smearing of the image may result.
U.S. Pat. No. 4,232,959 discloses a toner image fusing apparatus including a heated roller and a pressure roller, wherein a temperature sensor measures the surface temperature of the heated roller, and the contact pressure exerted by the pressure roller on the heated roller is then varied as a function of the measured surface temperature. These measures are taken in an attempt to achieve a uniform quality of fusion of the toner image on a copy sheet which is passed between the two rollers, since the process of fusing is influenced both by means of temperature and also by means of contact pressure between the two rollers. The pressure-temperature relationship is such that, for example, contact pressure is reduced when surface temperature rises and contact pressure is increased when surface temperature drops. In the process, the interrelation between these two variables is controlled on the basis of a predetermined relationship between surface temperature and contact pressure.
In the fusing apparatuses of the prior art, either a roller is used, in which the end portions are tapered in the outward direction and which is deformed under the action of contact pressure exerted by another roller, in order to obtain a favorable pressure profile over the length of the roller and thus prevent wrinkling; or contact pressure between the fusing roller and pressure roller is varied according to the measured temperature such that a predetermined relationship between pressure and temperature is maintained.